<!DOCTYPE html>
<html>
<head>
    <meta charset='utf-8'>
    <meta http-equiv='X-UA-Compatible' content='IE=edge'>
    <title>Page Title</title>
    <meta name='viewport' content='width=device-width, initial-scale=1'>
    <link rel='stylesheet' type='text/css' media='screen' href='main.css'>
    <script src='main.js'></script>
</head>
 <style>
    .box {
      width: 100px;
      height: 200px;
      border: 1px solid gray;
      z-index: 0; 
      float: left;
    }

    .box:hover {
      border: 3px solid red;
      z-index: 1; 
    }
  </style>
<body>
    <div class="box"></div>
    <div class="box"></div>
    <div class="box"></div>
    <div class="box"></div>
    <div class="box"></div>
    <div class="box"></div>
</body>
</html>